I know I'm so late to this thread, but as an active Vbulletin Admin with tons of experience with the reputation system, may I recommend one thing to you:
http://www.vbulletin.org/forum/showthread.php?t=165673
A simple, easy to customize mod. It adds a nice Thanks button to the bottom of posts, and you can set it to give +rep for every thank you received.
Also, if you wanna install it quickly, Stijn you have my MSN and I can tell you the phrases to edit to customize it so you could have it set up in less than 5 minutes.
I attached a photo of how we use it on Digiex.
Go on, install a mod on your otherwise plan vBulletin 
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-Nimrod
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